Process for oxidizing xylenes to phthalic acids

ABSTRACT

Continuous process for the oxidation of xylenes with oxygen at about 105* to 150* C wherein xylene is added at the same rate as phthalic acid is formed and xylene and partially oxidized intermediates, including those entrained in the phthalic acid are recycled continuously, a critical hold-up time is exceeded and the reaction mixture contains at least about 0.1 mole percent xylene based on the total of xylene and its oxidation products.

United States Patent Sullivan [151 3,700,731 [451 Oct. 24, 1972 [54]PROCESS FOR OXIDIZING XYLENES TO PHTHALIC ACIDS [72] Inventor: Robert H.Sullivan, Woodbury, NJ.

[73] Assignee: E. l. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: Feb. 28, 1969 [21] Appl. No.: 803,441

Related US. Application Data [63] Continuation-impart of Ser. Nos.667,646, Sept. 14, 1967, abandoned, and Ser. No. 742,150, July 3, 1968,abandoned.

[52] US. Cl. ..260/524 R [5 1] Int. Cl ..C07c 63/02 [58] Field of Search..260/524 R [56] References Cited UNITED STATES PATENTS 2,962,36111/1960 Spiller et a1 ..260/524 2,245,528 6/ 1941 Loder "260/ 524Fortuin et al ..260/524 Saffer ..260/5 24 Primary Examiner-Lorraine A.Weinberger Assistant Examiner-R. S. Weissberg Att0meyJames R. Morrison[5 7] ABSTRACT those entrained in the phthalic acid are recycledcontinuously, a critical hold-up time is exceeded and the reactionmixture contains at least about 0.1 mole percent xylene based on thetotal of xylene and its oxidation products.

10 Claims, 1 Figure PROCESS FOR OXIDIZING XYLENFS TO PHTHALIC ACIDSCROSS-REFERENCE TO PRIOR APPLICATIONS BACKGROUND OF THE INVENTION 1.Field of Invention This invention relates to the oxidation of alkyl sidechains of substituted aromatic compounds.

2. Description of the Prior Art Many methods have been disclosed for theoxidation of xylenes and their intermediate oxidation products withmolecular oxygen, particularly the air oxidation of p-xylene,p-tolualdehyde, or p-toluic acid to terephthalic acid. However, all ofthese methods have certain serious disadvantages. For example, theoxidation of p-xylene, alone, directly by an oxygen-containing gas inthe presence of a metal catalyst produces, almost exclusively, p-toluicacid, which then must be esterified in order to oxidize effectively theremaining methyl group. Phthalic acids have been made by oxidizingp-xylene in solution in an aliphatic acid which also contains a bromidepromoter and a metal catalyst (US. Pat. No. 2,833,816). Although thismethod does give phthalic acid in one step, it is not entirely suitablebecause the bromide increases the corrosivity of the oxidation medium.Another method for making phthalic acids involves adding a ketonepromoter, such as methyl ethyl ketone, to an oxidation system containingp-xylene, an aliphatic acid and a metal catalyst (US. Pat. No.3,036,122). This process involves making, as a co-product, a largequantity of acetic acid which must be sold in order for the process tobe economically attractive.

US. Pat. No. 2,723,994 discloses the unpromoted oxidation of a mixtureof xylene and toluic acid by oxygen wherein the ratio of acid to xyleneis maintained below 75:25 but above 50:50; however, the yields andconversions taught are relatively low and the conditions unsuitable forcontinuous operation.

SUMMARY OF THE INVENTION This invention provides an improved process foroxidizing xylenes to phthalic acids which gives high yields andspace-time yields and does not require a promoter. More specifically,there is provided in accordance with this invention an improvement inthe process for oxidizing xylenes, particularly p-xylene, to thecorresponding phthalic acids, in solution in the presence of heavy metaloxidation catalyst. The improvement of this invention comprises carryingout the oxidation reaction at a temperature of about from 105 to 150C,continuously feeding xylene, preferably p-xylene, to the reactionmixture at about the same rate phthalic acid is formed and continuouslyrecycling xylene plus oxidation intermediates soluble in the system atthe reaction conditions including those obtained on gross separation ofthe phthalic acid from the reaction mixture as well as suchintermediates entrained in the solid phthalic acid formed in thereaction. The minimum hold-up time (as defined hereinafter) of thereaction mixture is equal to the sum of the reciprocals of the firstorder rate constants for the oxidation reactions in the system and thexylene concentration is at least about 0.1 mole percent of the total ofxylene and oxidation products thereof, that is, tolualdehyde, toluicacid, formylbenzoic acid, and phthalic acid.

DETAILS OF THE INVENTION One preferred embodiment of this invention isschematically illustrated in the accompanying drawing.

In the drawing 2 represents a reactor fitted with an agitator. The feed1 is a mixture containing xylene, acetic acid, tolualdehyde, cobalt (asa soluble cobalt compound), toluic acid and fonnylbenzoic acid. Air isfed in at 9. The reaction mixture is withdrawn continuously (i.e.,steadily) or in frequent intervals during the reaction and sent to afilter 3 where the terephthalic acid is filtered 0E. The filtrate .8 isdistilled at 5 to remove water-of reaction 11 and some acetic acid. Theterephthalic acid is sent to a digester 4 where it is digested withacetic acid fed at 14., and filtered. The filtrate 7 is combined withfiltrate 8 and distilled at 5, the remainingsolution 12 of p-xylene andoxidized derivatives thereof. being recycled to reactor 2. Theterephthalic acid 13 can be sent to subsequent purification steps. Freshxylene is fed in at 10 to replace that converted by oxidation.

The process of this invention is particularly adapted to the oxidationof p-xylene to terephthalic acid although it is useful for otherxylenes, e. g., m-xylene to isophthalic acid, or to mixtures of xylenesto mixed phthalic acids. Unless otherwise indicated phthalic acid isused herein to refer generally to all the isomers of benzenedicarboxylic acid.

The heavy metal catalyst can be any of those conventionally used in theoxidation of xylenes with molecular oxygen, including those of the heavymetals shown on page 56 and 57 of Langes Handbook of Chemistry", 8thEdition, 1952. Preferably, the catalyst comprises a heavy metal havingan atomic number of 25-28 inclusive, and most preferably cobalt ormanganese. Mixtures of heavy metals can be employed also. The heavymetal can be introduced into the system in any form, the onlyrequirement being that it be soluble in the system or react with somecomponent of the system to become soluble. Thus, the heavy metal can bein the form of metal, oxide, or salt such as acetate, nitrate ornaphthenate. However the heavy metal catalyst is introduced be itsoluble per se or react to yield a compound which is soluble, theultimate form of the catalyst should be soluble at least to the extentnecessary to provide the desired quantity of active heavy metal, usuallyat least about 0.1 percent (calculated as metal) based on the weight ofthe reaction mixture, and preferably about from 0.2 to 5 percent.

The solvents used in accordance with this invention also can be thoseconventionally used in the oxidation of xylenes with molecular oxygen.Of these, 2 to 6 carbon aliphatic monocarboxylic acids such as acetic,propionic, butyric, valeric and caproic acids, as well as mixtures ofsuch solvents are usually used. Because of its low cost and its goodsolvent properties acetic acid is particularly preferred. Usually, theweight ratio of solvent to xylene and its oxidized derivatives,calculated as equivalent xylene (one mole of oxidized product equals onemole of xylene), is about from 1:1 to 12:1, a ratio of about 1.521 to3:1 being particularly preferred. If too much solvent is present, therate of oxidation is lowered, while if too little solvent is used thereaction mixture becomes difficult to agitate and contact with oxygen.

The reaction temperature is about from 105 to 150C and preferably 115 to140C. Below about 105C the oxidation rate is markedly slowed.Substantially above 140C solvent, e.g., acetic acid, and xylene aredegraded as evidenced by evolution of CO Also, at high temperature, sidereactions occur, such as thermal decomposition of hydroperoxide, whichgive byproducts, such as phenols, which are not only undesired, butpoison the oxidation.

Fresh xylene is added at a rate substantially equal to the rate offormation of phthalic acid. The recognition of the series ofintermediates through which xylene passes and necessity of their recyclewith the xylene feed is an important aspect of this invention. Thisseries is considered herein as:

xylene tolualdehyde toluic acid formylbenzoic acid phthalic acid whereinthe ks are the first order rate constants for the reactions involved.Although the reaction sequence just noted is simplified and otherintermediates are present along the reaction path in lesser amounts orin less stable configurations, the course of reaction can be defined bydetermining the concentrations of the five noted materials anddetermining reaction rate parameters from such concentrations.

It has been found that mere separation of the solid phthalic acid formedduring the reaction from the reaction mixture, e.g., by filtration, doesnot separate a large proportion of critical intermediates, particularlytoluic acid and formylbenzoic acid, which are entrained with thecrystals of the phthalic acid. Therefore, special steps must be taken torecover such entrained intermediates. Preferably, at least 50 percent byweight and usually at least 90 percent of such entrained intermediatesare recovered and recycled, preferably in a digestion step or bydissolution and recrystallization described below. Unless such entrainedintermediates are recycled to the reaction mixture, the unique resultsobtained in accordance with this invention are not obtained.

Another critical aspect of this invention is that the hold time in thereactor be at least equal to the sum of reciprocals of the first orderreaction rate constants under the conditions of reaction. As a roughapproximation, minimum hold time can be estimated as 1.16/k wherein k isthe first order reaction rate constant for the reaction of toluic acidto formylbenzoic acid under the reaction conditions. The hold time inany particular case also is equal to the moles of xylene and itsoxidation products in the reactor divided by the moles of fresh xylenefed per minute. Generally, hold time decreases with an increased oxygenpartial pressure and catalyst concentration. For the preferred ranges ofcatalyst concentration and oxygen partial pressure, the minimum holdtime is on the order of 1 15 minutes or higher, e.g., 140 minutes,depending on the aforementioned reaction conditions. i

The reaction mixture also contains at least about 0.1 mole percentxylene, and preferably at least 1 mole percent, based on xylene andoxidation products thereof. In efiect, this specifies a maximum holdtime for any particular feed rate. The process of this invention can becarried out in two or more stages in series with recycle between eachstage, or alternatively with recycle across the whole series of stages.In such case, the conditions set forth herein, particularly as to holdtime minimums apply to the average conditions, i.e., the conditionsacross the series of stages. Preferably, however, the process of thisinvention is carried out in a single stage reactor with conventionalmeans for vigorous agitation.

The process of this invention can be carried out with pure oxygen, air,oxygen-enriched air, or other mixtures of oxygen with inert gases suchas C0,, nitrogen, argon, etc. Air and air enriched with up to about 50percent of oxygen are particularly preferred because of rapidity ofoxidation and low cost. The partial pressure of oxygen is usually about1.5 to 45 psi, and preferably, for optimum speed of reaction andeconomics, 4.5 to 30 psi, based on non-condensibles. Conveniently, suchpartial pressure can be measured by correlation with total reactorpressure and oxygen content of the exit gas (less condensibles), e.g.,at 150 psia, 1.5 and 4.5 psi correspond to l and 3 percent oxygen,respectively, in the exit gas and at 300 psia, 30 and 45 psi correspondto 10 and 15 percent oxygen, respectively. Normally, the oxygen feedcorresponds to about 1.5 to 500, and preferably 1.5 to moles of oxygenper mole of methyl groups being oxidized. Of course, total pressurevaries with the particular system, and especially with the partialpressure of the solvent; the principal requirement for total pressure isthat the pressure be sufficient to keep all reactants in the liquidphase. Normally, such pressures vary from atmospheric to 200 atmospheresand, usually, atmospheric to 50 atmospheres.

Water is normally distilled from the liquid components of the reactionmixture during recycle (at 5 in the drawing), preferably in an amountsufiicient to hold the water content in the reaction mixture at lessthan 15 percent, and preferably less than 5 percent of the totalreaction mixture. Water of reaction also can be removed by flashing partof the liquid in the reaction product.

Under the preferred oxygen partial pressures set forth above and underthe other reaction conditions described herein, the weight ratios ofxylene to intermediate oxidation products in the continuous reactionmixture is about as follows:

Xylene to Wt. Ratio tolualdehyde l/l-S/l, e.g., 2/1-5/1 toluic acidl/l5-l/40, e.g., l/lS-l/3O formylbenzoic acid 0.25/1-2/1, e.g.,0.4/1-2/1 Oxidation intermediates entrained in the crystals of phthalicacid in the reaction mixture slurry can be removed by digestion atelevated temperature, e.g., 200 to 300C, preferably for about from 1 to30 minutes. Preferably, there is a staged separation of oxidationintermediates for recycle to the reactor. Thus, preferably, the bulk ofthe liquids in the reaction mixture are separated from the phthalic acidcrystals, e.g., by filtration or centrifuging, optionally with a washwith the solvent used for the reaction mixture, then digested atelevated temperature in a suitable solvent, e.g., 200 to 300C,preferably for l to 30 minutes, the digestion time varying inverselywith the temperature. The digested product is cooled, e.g., to l00l50C,to crystallize phthalic acid therefrom, the phthalic acid separatedtherefrom, e.g., by filtration or centrifuging, the filtrate containingentrained impurities being added to the filtrate from the firstseparation step. Solvents used in the digestionstep are preferably thoseused in the reaction, especially acetic acid, the weight ratio ofsolvent to phthalic acid usually being about from 1:1 to 20:1, andpreferably 2:1 to :1.

Instead of digesting the crude phthalic acid crystals after separatingthe bulk of the liquid therefrom, alternately the crude phthalic acidcan be dissolved in the solvent for the reaction mixture, thenrecrystallized and filtered or centrifuged, the separated liquidscontaining intermediate oxidation products being recycled to thereaction mixture. Preferably, such dissolution is carried out in aceticacid or concentrated (80 wt or more) aqueous solution thereof attemperatures on the order of 280 to 350C and preferably about 300C. Therecrystallization can be carried out in a single crystallization vessel,e.g., at 150 to 200C, or, alternately, the solution of crude phthalicacid can be passed through a plurality of, e.g., two or threecrystallization vessels in series, the pressure and temperature beinglet down between each vessel, the recrystallized acid being removed andfiltered or centrifuged from the product of the last of the stagedcrystallizers. The final crystallization temperature should, of course,be above that at which impurities, principally toluic acid, begin tocoprecipitate in substantial quantities. Also, various washes can beinterspersed between the aforementioned steps, e.g., an acetic acid oraqueous acetic acid wash of the crude acid prior to recrystallizationand similar acidic and/or water washes thereafter. Filtrates orcentrifugates containing significant quantities of intermediateoxidation products are recycled to the reaction medium.

As use dictates, the resulting semi-refined phthalic acid can be furtherpurified, e.g., to fiber-grade quality, as described, for example, inBritish Pat. No. 994,769.

The liquid components obtained through the separation stages arepreferably combined and distilled to remove water of reaction and excesssolvent, xylene and the. oxidation product intermediates thereofcontained in the liquid being combined with fresh xylene feed andreturned to the reactor.

In the following detailed working examples which illustrate thisinvention, parts and percentages are by weight unless otherwiseindicated. All equipment is either glass lined or of stainless steel,except the digester which is glass or titanium lined.

An autoclave is charged about two-thirds full with a portion of amixture comprising 1,000 parts of acetic acid, 450 parts of p-toluicacid, 150 parts of p-xylene, 925 parts of cobalt acetate tetrahydrateand 25 parts of water, pressurized to 300 psig with nitrogen and heatedto 130C. Vigorous agitation is started, then air is fed to the bottom ofthe autoclave. Initially, oxygen content monitored in the off gas fallsto about 2 to 5 percent oxygen then rises and is maintained at about 5to 7 percent oxygen. (Oxygen partial pressure about 15 to 20 psi basedon non-condensibles). The temperature is maintained at 130C by cooling.The above mixture is used as feed until recycle is obtained andaccumulated in a hold tank as feed as described below, at which time therecycle plus fresh p-xylene gradually replace the start-up feed. Atsteady state conditions the fresh p-xylene and recycle are combined andcontinuously fed to the reactor, the fresh p-xylene being added at aboutthe same rate terephthalic acid is formed. Gas and slurry product arewithdrawn continuously from a side arm of the reactor. The gaseouseffluent is vented through a water-cooled condenser while the slurryreaction mixture is collected at about 100C in a holdup tank for thetwo-stage separation of terephthalic acid from reaction intermediates.

Slurry is withdrawn from the hold tank and filtered at atmosphericpressure at about 100C, the crude terephthalic acid being washed with asmall proportion of acetic acid and sent to the subsequent purificationwhile the filtrate including the wash acid is distilled at atmosphericpressure to remove water of reaction overhead and excess acetic acid asan intermediate cut. Unreacted xylene carried overhead with the water isseparated from the water and returned to the reactor with the recyclefeed.

The crude filter cake from the first stage separation, containing aboutterephthalic acid, 10 percent ptoluic acid and 5 percent p-formylbenzoicacid (dry basis) is slurried with acetic acid in about a 2:1 weightratio then digested at about 240C for 10 minutes at autogenous pressure(500-520 psig). The digested product is filtered at about C atatmospheric pressure, washed with acetic acid and air dried to constantweight at 100C to yield semi-refined terephthalic acid of about 98-99percent purity in a yield in excess of 95 percent based on p-xylene.

The filtrate and wash liquor from the second stage separation arecombined and evaporated to dryness at a pressure of 26 inches of mercuryto yield solid p-toluic acid, p-formylbenzoic acid, and associatedintermediates which are incorporated in the recycle feed. Alternately,the filtrate can be combined directly with the filtrate from first stageseparation, then the combination distilled to remove water and excessacetic acid and recycled.

The steady-rate continuous reaction conditions are:

Temp, C,

Pressure, psig, 300

Hold-up time, 230 minutes Minimum hold-up time under reactionconditions,

about 3 mole 5 based on C8 components inclusive of terephthalic acid.

In a like manner isophthalic acid can be prepared by substituting anequal weight of m-xylene for the pxylene in the above example.

EXAMPLE 2 REACTOR Temp., C 130 Pressure:

Total pressure, psig 225 0, partial pressure, psi 23.5 Hold time, min250 Minimum hold time under Conditions, min 140 Air Feed Rate 0.22part/part total feed Liquid feed rate, parts by wt 28 parts/min FeedComposition,

Acetic Acid 70.5 p-Xylene 4.8 p-Tolualdehyde 0.3 p-Toluic Acid 18.9p-Formylbenzoic Acid 1.0 Terephthalic Acid 0.4 Cobalt acetatetetrahydrate (as Co.) 0.92 Water 3.2 Reactor Composition,

Acetic Acid 67.7 p-Xylene 0.6 p-Tolualdehyde 0.3 p-Toluic Acid 17.4p-Formylbenzoic Acid 1.2 Terephthalic Acid 7.6 Cobalt 0.89 Water 4.3 OffGas, O ICO 9.8/0.8

DIGESTER Temperature, C 240 Pressure, psig 500520 Digestion time, min 10Product Composition,

Terephthalic Acid 98.9 p-Toluic Acid 0.4 p-Formylbenzoic Acid 0.7Overall yield, p-xylene to terephthalic acid 95% At least equivalentresults are obtained if, instead of catalyst, the improvement whichcomprises maintain- I The above procedure is repeated at a reactiontemperature of C, using oxygen at a partial pressure of 10 psi. Similarresults are obtained except for a slightly lower space-time-yield.

I claim:

1. In the process for the manufacture of phthalic acids which comprisesoxidizing xylenes in solution with molecular oxygen in the presence ofheavy metal ing the reaction mixture at a temperature of about from 105to 150C, continuously feeding xylene to the reaction mixture at aboutthe same rate phthalic acid is formed, continuously recycling xylene andpartially oxidized derivatives thereof, including at least 50 percent byweight of those entrained in the solid phthalic acid as it is formed,the minimum hold-up time in said reaction mixture being equal to the sumof the reciprocals of the first order rate constants of the oxidationreactions at the reaction conditions, the xylene concentration being atleast about 0.1 mole percent of the total of xylene and oxidationproducts thereof.

2. A process of claim 1 wherein p-xylene is oxidized to terephthalicacid, solid terephthalic acid being separated from the liquid componentof the reaction mixture, p-xylene and oxidized derivatives thereof soseparated as well as such products entrained in said separateterephthalic acid being continuously recycled to said reaction mixturealong with fresh p-xylene.

3. A process of claim 2 wherein a solution of pxylene in acetic acid isoxidized in a single stage with molecular oxygen at a partial pressureof about from 1.5 to 45 psi in the presence of a cobalt catalyst.

4. A process of claim 3 wherein terephthalic acid separated from thereaction mixture is digested in acetic acid to remove entrained partialoxidation products which are recycled to the reaction mixture.

5. A process of claim 4 wherein said terephthalic acid separated fromthe reaction mixture is digested in about 1 to 20 times its weight ofacetic acid at about from 200 to 300C for about from 1 to 30 minutes toremove intermediates entrained therein.

6. A process of claim 3 wherein terephthalic acid separated from thereaction mixture is recrystallized from acetic acid or a concentratedaqueous solution thereof to remove entrained partial oxidation productswhich are recycled to the reaction mixture.

7. A process of claim 3 wherein the minimum quantity of p-xylene in saidreaction mixture is about 1 mole percent based on the total of p-xyleneand oxidation products thereof.

8. A process of claim 7 wherein the oxygen partial pressure is aboutfrom 4.5 to 30 psi, the amount of cobalt catalyst is at least about 0.1percent by weight of the reaction mixture calculated as cobalt and theholdup time is at least about 1 15 minutes.

9. A process of claim 8 wherein the weight ratio of acetic acid to thetotal of p-xylene and oxidation products thereof in the reaction mixtureis about from 1:1 to 3:1.

10 A process of claim 9 wherein said reaction tembeing digested, thecrude filter cake iswashed with 65 Peramreisaboutfmm 1150) acetic acid,dissolved therein at about 300C, recrystal-

2. A process of claim 1 wherein p-xylene is oxidized to terephthalicacid, solid terephthalic acid being separated from the liquid componentof the reaction mixture, p-xylene and oxidized derivatives thereof soseparated as well as such products entrained in said separateterephthalic acid being continuously recycled to said reaction mixturealong with fresh p-xylene.
 3. A process of claim 2 wherein a solution ofp-xylene in acetic acid is oxidized in a single stage with molecularoxygen at a partial pressure of about from 1.5 to 45 psi in the presenceof a cobalt catalyst.
 4. A process of claim 3 wherein terephthalic acidseparated from the reaction mixture is digested in acetic acid to removeentrained partial oxidation products which are recycled to the reactionmixture.
 5. A process of claim 4 wherein said terephthalic acidseparated from the reaction mixture is digested in about 1 to 20 timesits weight of acetic acid at about from 200* to 300*C for about from 1to 30 minutes to remove intermediates entrained therein.
 6. A process ofclaim 3 wherein terephthalic acid separated from the reaction mixture isrecrystallized from acetic acid or a concentrated aqueous solutionthereof to remove entrained partial oxidation products which arerecycled to the reaction mixture.
 7. A process of claim 3 wherein theminimum quantity of p-xylene in said reaction mixture is about 1 molepercent based on the total of p-xylene and oxidation products thereof.8. A process of claim 7 wherein the oxygen partial pressure is aboutfrom 4.5 to 30 psi, the amount of cobalt catalyst is at least about 0.1percent by weight of the reaction mixture calculated as cobalt and thehold-up time is at least about 115 minutes.
 9. A process of claim 8wherein the weight ratio of acetic acid to the total of p-xylene andoxidation products thereof in the reaction mixture is about from 1:1 to3:1.
 10. A process of claim 9 wherein said reaction temperature is aboutfrom 115* to 140*C.